<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      //   var a = "Jay Chou love you";
      
      function fn(a){
        var c = "";
      if (a.length <= 16 && a.length >= 4) {
        for (var i = 0; i < a.length; i++) {
          if (!isNaN(parseInt(a[0]))) {
            var c='不合格'
           return c;
          } else if (
            (a.charAt(i) >= "A" && a.charAt(i) <= "Z") ||
            (a.charAt(i) >= "a" && a.charAt(i) <= "z") ||
            (a.charAt(i) >= "0" && a.charAt(i) <= "9") ||
            a.charAt(i) !== "_"
          ) {
     
            var c='合格'
           return c;
          } else {
           
            var c='不合格'
           return c;
          }
        }
      } else {
       
        var c='不合格'
           return c;
      }

      }
      console.log(fn("JayChou13"))
      
    </script>
  </body>
</html>
